0013-4651/2010/157 7 /A841/5/$28.00 The Electrochemical Society Nanoparticle Nanorod Core Shell LiNi 0.5 Mn 1.5 O 4 Spinel Cathodes with High Energy Density for Li-Ion Batteries Minki Jo, a Young-Ki Lee, b Kwang Man Kim, b and Jaephil Cho a, *,z a School of Energy Engineering and Converging Research Center for Innovative Battery Technologies, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea b Research Team of Nano-Convergence Sensor, Electronics and Telecommunications Research Institute, Daejon 305-700, Korea A841 Nanoparticle nanorod core shell LiNi 0.5 Mn 1.5 O 4 spinel cathodes for Li-ion batteries were prepared using a hollow MnO 2 precursor. The core and shell parts consisted of nanoparticle 100 nm and nanorod assemblies, respectively. The core shell cathode exhibited greatly improved discharge capacities compared to nanoparticles prepared by a sol gel method. The core shell spinel exhibited discharge capacities of 121 and 100 mah/g at 0.1C and 7C rates, respectively, whereas a spinel cathode prepared by a sol gel method exhibited 99 and 80 mah/g at those respective rates. In addition, the core shell spinels demonstrated an energy density value that was enhanced by 52% to 1.6 Wh/cm 3 compared to an analogous sample prepared by a sol gel method, which showed a value of 0.9 Wh/cm 3. 2010 The Electrochemical Society. DOI: 10.1149/1.3428706 All rights reserved. Manuscript submitted March 4, 2010; revised manuscript received April 12, 2010. Published May 25, 2010. Recently, Li-ion batteries with a high rate capability have received much attention from researchers due to their uses in power tools and hybrid electric vehicles. 1-3 To fulfill these usages, among the various cathode materials, LiMn 2 O 4 spinel has received much attention due to its good thermal stability and fast Li-ion diffusivity. 4-8 However, dissolution of Mn 3+ from the Li x Mn 2 O 4 spinel structure leads to the destruction of the structure initiating at the electrolyte and electrode interfaces, resulting in rapid capacity fade. 9,10 To improve this structural instability, substitution of transition metals M = transition metals into the Mn sites in LiM x Mn 2 x O 4 has been investigated intensively. 11-15 Among the transition metals, LiNi 0.5 Mn 1.5 O 4 spinel exhibited an additional 4.7 V plateau due to the presence of a Ni 2+/ Ni 4+ redox pair, thus delivering 100 mah/g reversible capacity. 13,16,17 To meet the rate characteristics of the batteries, although cell design is an important factor that affects rate capability power, the most essential factor that determines the rate is the electrode material. The purpose of these materials is to achieve a decrease in the diffusion length of the Li ion in the electrode particle, thus decreasing the kinetic barrier and in turn reducing the polarization factor at higher C rates. One means of achieving high rate capability is to manipulate the particle morphology into mesoporous nanowires, coating the materials with high electronic conducting materials. 18 However, these bulky nanoparticles lead to a decreased energy density Wh/cm 3 due to the much lower electrode density as the nanoparticles require higher amounts of binder and conducting agent compared to the bulk particles. In consequence, the best approach to improve the rate capability without losing the electrode density is to use clustered nanoparticles with a bulk size of 5 m. In this study, the synthesis and electrochemical properties of nanoparticle nanorod core shell LiNi 0.5 Mn 1.5 O 4 spinel particles with a high energy density and high power were investigated using a hollow nanowire MnO 2 cluster precursor. The core shell showed a reversible capacity of 121 mah/g and a capacity retention ratio of 83% even at a cycling rate of 7C. Moreover, its power density was improved by 52% compared to analogous nanoparticles prepared by a sol gel method. Experimental To prepare hollow MnO 2 nanorod clustered precursors, a 1:1 molar ratio of MnSO 4 5H 2 O and NH 4 2 S 2 O 8 was dissolved in distilled water and transferred into the autoclave and then reacted for 12 h at 120 C. After the reaction, the mixture was rinsed five times * Electrochemical Society Active Member. z E-mail: jpcho@unist.ac.kr with distilled water and was vacuum-dried at 120 C. An as-prepared MnO 2 template was used to prepare LiNi 0.5 Mn 1.5 O 4, and appropriate amounts of LiC 2 H 3 O 2 H 2 O and Ni NO 3 2 6H 2 O were completely dissolved in distilled water and mixed with MnO 2 in a molar ratio of 1:0.5:1.5 LiC 2 H 3 O 2 H 2 O:Ni NO 3 2 6H 2 O:MnO 2. After drying it in an oven, the product was ground using mortar and pestle, followed by firing at 400 and 700 C for 2 and 8 h, respectively. For comparison, the cathode powder was obtained using the sol gel process, LiC 2 H 3 O 2 H 2 O was first dissolved in distilled water, and then MnC 2 H 3 O 2 4H 2 O and Ni NO 3 2 6H 2 O were continuously dissolved into it with a molar ratio of 1:1.5:0.5 Li:Mn:Ni. Finally, poly vinylpyrrolidene as a chelating agent was mixed together, and NH 4 OH was added to adjust ph 8.5 to 9.0. The solution was dried at 90 C and fired at 850 C for 10 h. To test the cycle-life performance of each cathode material, a slurry was prepared by mixing the active material, Super P carbon black, and a poly vinylidene fluoride binder with weight ratios of 60:20:20 in N-methyl-2-pyrrolidene in a homogenizer Youngjin Corp., Korea. Electrode density was measured under porosities of 20 23%. The pore volume of the electrodes was obtained with a mercury porosimeter Micromeritics Pore Sizer 9320. To adjust for such volume fractions, the electrode composition was adjusted to the electrode composition described above, and the roll-pressing frequency was varied. A coin-type half-cell 2016 size contained a test electrode, a lithium-metal counter and reference electrodes, a 15 m thick microporous polyethylene separator, and an electrolyte solution of 1 M LiPF 6 in ethylene carbonate/dimethyl carbonate 1:1 vol % LG Chem., Korea. Cell tests were carried out at various C rates under 21 and 60 C. Powder X-ray diffraction XRD, D/MAX-2200V, Rigaku measurements using Cu K radiation were employed to identify the crystalline phase of the synthesized materials. A field-emissionscanning electron microscope NanoSem 230 and a transmission electron microscope TEM JEM-2100 were used to assess the morphology of the obtained samples. Cross-section images were obtained with a focused ion beam Quanta 3D FEG using a Ga source. Electrochemical impedance spectroscopy EIS data were collected before and after 20 cycles at a rate of 7C after discharging to 3.5 V with an ac amplitude of 10 mv in the frequency range of 0.5 MHz 10 mhz by an Ivium impedance analyzer. The specific surface areas of the samples were measured using N 2 physisorption at 196 C by the Brunauer Emmett Teller BET method using an automatic surface analyzer Quantachrome Autosorbs-1. Results and Discussion The XRD pattern of the MnO 2 precursor indicates the formation of the -MnO 2 tetragonal phase with a minor -MnO 2 phase Fig.
A842 Figure 1. XRD patterns of the MnO 2 precursor and the core shell LiNi 0.5 Mn 1.5 O 4 spinel. 1. Its XRD pattern exhibits the formation of the cubic spinel Fd3m phase the lattice constant was 8.170 6 Å and a minor NiO impurity phase. Figure 2a shows scanning electron microscopy SEM images of MnO 2, which consist of nanowire clustered particles with a wire length of 3 m. In particular, the crosssectioned image of the powder Fig. 2b shows a hollow urchinlike morphology with three different regions of hollow, porous, and dense. Figures 3a and b show SEM images of LiNi 0.5 Mn 1.5 O 4 spinels obtained using a MnO 2 precursor after annealing. The hollow region of the precursor was filled with 100 nm nanoparticles that are directly adjacent to the nanorod clusters its morphology is quite similar to a sunflower. The high resolution TEM image Fig. 3d clearly shows the presence of the lattice fringe of the 111 plane of the spinel, corresponding to a d-spacing value of 4.7 Å; the inset of the selected area diffraction pattern Fig. 2f of the spinel particle confirms the single-crystalline phase. Overall, the core shell spinel particle size ranges from 7 to 10 m with a spherical-like morphology Fig. 4a, and even after electrode pressing, the core shell morphology appears to be maintained Fig. 4b. For spinel particles prepared by the sol gel method, 1 m particles resulted see Fig. 5. The BET surface area of the hollow spinel particles prepared by the MnO 2 precursor was 17 m 2 /g, whereas that prepared by sol gel was 9 m 2 /g. The BET surface area of MnO 2 precursors was 45 m 2 /g. Figure 6 shows voltage profiles of the spinels obtained from the MnO 2 template and a sol gel method as a function of the cycle number with C rates increasing from 0.1C to 7C identical charge and discharge rates were used. Figure 7 shows the discharge capacity of the spinels obtained from the MnO 2 precursor and a sol gel method as a function of the cycle number with C rates increasing from 0.1C to 7C identical charge and discharge rates were used between 5 and 3.5 V in coin-type half-cells. The core shell cathode shows first discharge capacities of 121 and 100 mah/g at rates of Figure 3. a SEM and b FIB cross-section images of the core shell LiNi 0.5 Mn 1.5 O 4 spinel prepared by hollow MnO 2 ; c and d TEM images of the core shell LiNi 0.5 Mn 1.5 O 4 spinel prepared by hollow MnO 2 an inset of d is selected area diffraction pattern of d. 0.1C and 7C, respectively, with capacity retention at 7C of 83%. Furthermore, there is no capacity fade under continuous cycles at 7C up to 20 cycles see Fig. 7. The spinel cathode prepared by the sol gel method exhibits a similar trend to the core shell particles, and its capacities were 99 and 80 mah/g, respectively, at the two aforementioned rates. The irreversible capacity ratios of the LiNi 0.5 Mn 1.5 O 4 electrode prepared by the sol gel method and those of the hollow MnO 2 precursor are 36 and 25%, respectively, which correspond to 67 and 75% coulombic efficiencies, respectively. (a) (b) Figure 2. Color online a SEM and b FIB cross-section images of the MnO 2 precursor. Figure 4. SEM images of a the core shell LiNi 0.5 Mn 1.5 O 4 spinels obtained using a MnO 2 precursor and b the core shell LiNi 0.5 Mn 1.5 O 4 spinel composite electrode after roll-pressing.
A843 Figure 5. SEM images of the LiNi 0.5 Mn 1.5 O 4 spinels obtained by sol gel method. After 7C rate cycling, the cells were run again at a rate of 0.1C, and both cathodes were recovered to full capacities Fig. 7a. This performance of the core shell sample is comparable to metal-oxidecoated LiNi 0.42 Mn 1.5 Zn 0.08 O 4, which showed 85% capacity retention at a rate of 7C in this test, the charge rate was unspecified. 19 However, the uncoated cathode showed rapid capacity fading at a rate of 7C of 50%. A spinel nanoparticle prepared by poly ethylene glycol as a chelating agent showed a particle size of 79 nm that had a reversible capacity of 98 mah/g upon rate cycling at 8C. 20 Compared to previous studies in which the LiNi 0.5 Mn 1.5 O 4 spinel particle size was 1 m the particle size is similar to our sample prepared by a sol gel method, 20 the electrode density is expected to be much smaller than the core shell spinels. The spinel particles prepared by a sol gel method had an electrode density of 1.9 g/cm 3, while the core shell particles had a density of 2.9 g/cm 3. An average electrode thickness and a pure loading amount of the spinel particles obtained from the sol gel method were 20 m and 7.2 mg, respec- Figure 6. Color online Voltage profiles of the LiNi 0.5 Mn 1.5 O 4 spinels prepared by a sol gel and b hollow MnO 2 precursor. C rate was increased from 0.1C to 7C and decreased to 0.1C identical charge and discharge rates were used between 5 and 3.5 V in coin-type half-cells. Figure 7. Color online Discharge capacity and energy density vs cycle number in the core shell particles prepared by hollow MnO 2 precursor and nanoparticles prepared by sol gel method at different C rates 1C corresponds to 140 ma/g. tively the electrode dimension was 1.9 cm 2. However, those from MnO 2 precursors were 15 m and 8.3 mg, respectively. Porosity ranged between 20 and 23%. When these were converted to the volumetric energy density the volumetric energy density was calculated from just the active material, the core shell particles showed 1.6 Wh/cm 3, while those prepared by the sol gel method showed 0.9 Wh/cm 3. Accordingly, the core shell spinels demonstrated a 78% enhanced electrode density value compared to the analogy prepared by the sol gel method. After 7C rate cycling, the core shell showed a value of 1.3 Wh/cm 3, whereas nanoparticles prepared by the sol gel method showed a value of only 0.7 Wh/cm 3 Fig. 7b. All Ni-containing spinel cathodes show voltage plateaus at 4.7 V associated with the Ni 2+ to Ni 4+ redox reaction. 13,16,17 In the ideal LiNi 0.5 Mn 1.5 O 4 structure, the oxidation state of manganese is fixed at +4. As the calcination temperature is increased to 700 C, however, an oxygen deficiency appears in LiNi 0.5 Mn 1.5 O 4, which partly lowers the manganese oxidation state from Mn 4+ to Mn 3+. 21 This implies that a small portion of the manganese remains as Mn 3+ for the LiNi 0.5 Mn 1.5 O 4 cathode. Figure 8 shows differential capacity curves of the first cycle at a rate of 0.1C in spinel powder prepared by the sol gel method. The two dominant peaks at 4.67 and 4.73 V are due to the Ni 2+ to Ni 4+ redox reactions. 21 These two peaks are associated with oxygen-deficient LiNi 0.5 Mn 1.5 O 4 with a space group of Fd3m face-centered cubic. However, the core shell spinel nanoclusters show an additional peak at 4.7 V, which is associated with the presence of minor LiNi 0.5 Mn 1.5 O 4 with a space group of P4 3 32 primitive simple cubic. 22 Compared to LiNi 0.5 Mn 1.5 O 4, which has a one-step phase transition between two cubic phases, LiNi 0.5 Mn 1.5 O 4 with two-step phase transitions between three cubic phases should have shown greatly deteriorated capacity retention at higher C rates due to the larger degree of strain generated by the high rate cycling. However, the result in this study showed no capacity fade at the 7C rate, indicating that a nanostructure with a higher surface area may accommodate the strains during phase transitions. In addition, the differential capacity profiles of both spinel electrodes clearly exhibit 4 V plateaus. This plateau is related to the redox reactions between
A844 (a) (b) Figure 8. Color online Differential capacity curves for the LiNi 0.5 Mn 1.5 O 4 spinels prepared a by sol gel method and b by hollow MnO 2 precursor. Mn 3+ and Mn 4+. With increasing nickel content in LiNi x Mn 2 x O 4, the intensity of the 4.1 V plateau gradually decreases conversely due to the decreased amount of Mn 3+. Because both spinel samples contain an impurity of NiO, the presence of the plateau at 4.1 V is reasonable. Figure 10. Color online a Discharge profiles of the LiNi 0.5 Mn 1.5 O 4 spinel cathode prepared by hollow MnO 2 precursor in coin-type half-cell at different C rates under 60 C identical charge and discharge rates were used. b Plot of discharge capacity vs cycle number of a. Figure 9. Color online Impedance spectra of the LiNi 0.5 Mn 1.5 O 4 spinels prepared a by sol gel method and b by hollow MnO 2 precursor. An inset of a is an equivalent circuit that was used to interpret the impedance spectra. To see any interfacial impedance changes between the two samples, EIS data were collected before and after 20 cycles at a rate of 7C after discharging to 3.5 V, as shown in Fig. 9. An equivalent circuit was used to interpret the impedance results inset of Fig. 9. R o is the ohmic resistance of the cell, and its value is 10 in both electrodes. R ct is the charge-transfer resistance at the electrode and electrolyte, and R sf is the surface film resistance, which correspond to a semicircle at high frequency. Before cycling, R ct and R sf in the spinel electrode obtained from the MnO 2 precursor are quite smaller than those obtained from the sol gel method. After cycling, the resistance of the cathode electrode obtained from the sol gel method is 580, while that obtained from the MnO 2 precursor is 429. This impedance spectra result shows that a nanostructured core shell spinel lends faster Li-ion and electron transport at the interfaces between the electrolytes and the electrode. Finally, the preliminary cycling test results of the core shell spinel cathode at 60 C are presented in Fig. 10. The capacity decay of the core shell spinel cathode was not severe compared to that of the LiMn 2 O 4 -type spinel 8 because most redox reactions come from Ni 2+ and Ni 4+. No dissolution of Mn and Ni ions in the electrolytes obtained from the cells after cycling at room temperature was observed based upon inductively coupled plasma mass spectroscopy analysis. However, the amount of Mn ion dissolution at 60 C was 30 ppm, and this amount did not result in fast capacity fade in LiMn 2 O 4 -type spinels dissolution of Ni ions was not detected at 60 C. 8 Hence, it is expected that accelerated electrolyte oxidations at 5 V might lead to capacity fade.
A845 Conclusions In summary, nanoparticle nanorod core shell LiNi 0.5 Mn 1.5 O 4 spinel cathodes for Li-ion batteries were successfully prepared using amno 2 precursor with identical morphology. The cathodes demonstrated a reversible capacity of 100 mah/g at a rate of 7C and no capacity fading. In addition, the energy density of the core shell particles was 1.56 Wh/cm 3, which was an improvement of 56% over nanoparticles prepared by a sol gel method. Acknowledgment This research was supported by the Converging Research Center Program through the National Research Foundation of Korea NRF funded by the Ministry of Education, Science and Technology 2009-0082083. Also, partial financial support from WCU program is acknowledged. Ulsan National Institute of Science and Technology assisted in meeting the publication costs of this article. References 1. D. Deng, M. G. Kim, J. Y. Lee, and J. Cho, Energy Environ. Sci., 2, 818 2009. 2. M. G. Kim and J. Cho, Adv. Funct. Mater., 19, 1497 2009. 3. R. F. Nelson, J. Power Sources, 92, 2 2001. 4. M. M. Thackeray, P. J. Johnson, L. A. Depicciotto, P. G. Bruce, and J. B. Goodenough, Mater. Res. Bull., 19, 179 1984. 5. M. M. Thackeray and A. Dekock, J. Solid State Chem., 74, 414 1988. 6. J. Cho and M. M. Thackeray, J. Electrochem. Soc., 146, 3577 1999. 7. J.-Y. Luo, Y.-G. Wang, H.-M. Xiong, and Y. Xia, Chem. Mater., 19, 4791 2007. 8. S. Lim and J. Cho, Chem. Commun. (Cambridge), 2008, 4472; Electrochem. Commun., 10, 1478 2008 ; J. Cho, J. Mater. Chem., 18, 2257 2008. 9. Y. Xia and M. Yoshio, J. Electrochem. Soc., 143, 825 1996. 10. R. J. Gummow, A. de Kock, and M. M. Thackeray, Solid State Ionics, 69, 59 1994. 11. A. D. Robertson, S. H. Lu, W. F. Averill, and W. F. Howard, Jr., J. Electrochem. Soc., 144, 3500 1997. 12. K. Amine, H. Tukamoto, H. Yasuda, and Y. Fujia, J. Electrochem. Soc., 143, 1607 1996. 13. Q. Zhong, A. Bonakdarpour, M. Zhang, Y. Gao, and J. R. Dahn, J. Electrochem. Soc., 144, 205 1997. 14. H. Shigemura, H. Sakaebe, H. Kageyama, H. Kobayashi, A. R. West, R. Kanno, S. Morimoto, S. Nasu, and M. Tabuchi, J. Electrochem. Soc., 148, A730 2001. 15. P. Arora, B. N. Popov, and R. E. White, J. Electrochem. Soc., 145, 807 1998. 16. H. Kawai, M. Nagata, H. Tukamoto, and A. R. West, J. Power Sources, 81 82, 67 1999. 17. T. Ohzuku, S. Takeda, and M. Iwanaga, J. Power Sources, 81 82, 90 1999. 18. K. M. Shaju and P. G. Bruce, Dalton Trans., 40, 5471 2008. 19. J. Liu and A. Manthiram, J. Electrochem. Soc., 156, A66 2009. 20. J. C. Arrebola, A. Caballero, M. Cruz, L. Hernan, J. Morales, and E. R. Castellon, Adv. Funct. Mater., 16, 1904 2006. 21. Y. Terada, K. Yasaka, F. Nishikawa, T. Konishi, M. Yoshio, and I. Nakai, J. Solid State Chem., 156, 286 2001. 22. J. H. Kim, S. T. Myung, C. S. Yoon, S. G. Kang, and Y. K. Sun, Chem. Mater., 16, 906 2004.